Republicans must win two seats now held by Democrats Tuesday to take over the state House of Representatives. That’s not likely to happen, but that isn’t stopping them from trying.

Democrats held a 34- to 31-seat advantage over Republicans in the 2015-16 session. That’s down from the 37-to-28 lead Democrats held in the 2013-14 session, the result of a Republican landslide election statewide in 2014 that narrowed the gap. If Republicans pick up two seats, they would hold a 33-32 advantage.

All 65 seats in the House are up for election to two-year terms. Most are safe seats to which incumbents will return with little effort. But 10 lawmakers hit term limits and can’t run again, leaving open races. Of the 10, eight are held by Democrats. While this might seem to give Republicans an opening, most of those districts are solidly Democratic.

Compounding the challenge for Republicans: Voter registration. This year, for the first time in more than three decades, statewide voter registration tipped to Democrats, and that could make it difficult for Republicans to wrest away the leadership.

So what are the races to watch in this battle to claim the House? Follow the money and it leads to eight seats, all in swing districts. Let’s look at the six most competitive.

The races

House District 33: Broomfield

Democratic Rep. Dianne Primavera is one of the 10 term-limited legislators, which leaves an open seat for which Republican Karen Nelson, a long-time legislative aide, and Democrat Matt Gray, a former deputy district attorney, are battling.

Gray has raked in far more contributions than Nelson with $133,577 to Nelson’s $20,366. He has also outspent her by the same percentage. That doesn’t always matter: in 2010, Primavera lost to Republican Don Beezley despite her fundraising advantage.

What’s notable about this seat: House District 33 is a rare species in our gerrymandered times. It’s one of only two open seats that actually lie within a competitive district. It’s flipped between Democrats and Republicans three times in the past four elections, and every race has been decided by just a few hundred votes.

House District 59: Durango

This is among the seats Republicans hope to hang onto. The race is a rematch of sorts between incumbent Republican J. Paul Brown of Ignacio and Democrat Barbara McLachlan of Durango. McLachan’s husband, Mike, defeated Brown for the seat in 2012. And then in 2014, Brown reclaimed it. Now, Barbara McLachlan seeks to take it back.

It’s also the second most expensive House race in the state, as of Oct. 26., McLachlan has raised $159,249 to Brown’s $152,569. McLachlan has spent less than Brown; as of Oct. 26 she had shelled out $133,343 to Brown’s $155,543.

What makes this race interesting? Besides the husband-wife tag team? Spending by outside groups. This has been the number one target of a Democratic-leaning independent expenditure committee, Common Sense Values, which has spent more than $500,000 on this race out of its nearly $3.4 million in expenditures.

There hasn’t been equivalent big-dollar spending on the Republican side by any single committee for House District 59; advertising and canvassing has been paid for by a half-dozen committees with much smaller budgets.

Democratic political consultant Ellen Dumm said if Democrats manage to pick up seats now held by Republicans, this is one race where it might happen, in part because Brown hasn’t been as moderate as the district might want.

This is the other rare competitive district with an open seat, and so far has been the most expensive House race in the state.

Democrat Jeff Bridges of Greenwood Village is competing against Republican Katy Brown of Cherry Hills Village. Both candidates had to survive strong primary challenges in June.

The race has been marked by lots of contributions from outside the district to Bridges, the son of millionaire Rutt Bridges. The elder Bridges was one of four Democratic millionaires who helped fund the 2004 takeover of the General Assembly by Democrats.

Brown, a member of the Cherry Hills Village City Council, won a prized endorsement from the Colorado Education Association; she’s the only Republican candidate, House or Senate, to earn the endorsement, and it has meant contributions from teachers’ unions. However, she still substantially trails Bridges in fundraising and spending.

Bridges, who previously worked as a congressional aide to U.S. Rep. John Salazar and U.S. Sen. Ken Salazar, has so far raised $227,510 to Brown’s $115,882. Bridges has outspent Brown as well, $219,915 to $109,881.

What makes the race interesting? Voter registration. Democrats hold an advantage of about 1,300 over Republicans in active voter registrations in this district of 15,000 voters. But who wins the seat will depend on who has the best appeal with unaffiliated voters, who lead by more than 1,500 in voter registration over Democrats and about 3,000 more than Republicans.

House District 17, Colorado Springs

Exum leads Roupe in both fundraising and spending, but it’s outside groups that have targeted both candidates that make the difference. Common Sense Values has put out pieces opposing Roupe, and Colorado Citizens for Accountable Government has done the same on Exum.

Roupe recently blasted one of the mailers that claimed she had called for an investigation of Planned Parenthood, which she said is untrue. Roupe was one of the few Republican lawmakers who expressed sympathy at the time for the victims of the November 2015 shooting at the Planned Parenthood clinic in Colorado Springs.

What makes this race one to watch? Voter registration and spending by outside groups. Only two House districts in El Paso County have more Democrats than Republicans, and this is one of them. Democrats lead Republicans by more than 2,000 registrations, but unaffiliated voters top both of them and by a fairly big margin.

Colorado State University political science professor and longtime Capitol observer John Straayer predicts a very close race, with Roupe one of the two most at-risk Republican incumbents. Dumm says it will all come down to turnout. “They don’t vote down there,” she said.

House District 30, Commerce City

Republicans face a battle holding on to this seat, now held by Rep. JoAnn Windholz of Commerce City. Windholz won the 2014 election by just 106 votes, over incumbent Democratic Rep. Jenise May.

But Windholz became famous, or infamous, depending on your perspective, by comments blaming Planned Parenthood for the Nov. 2015 shooting.

Windholz’ Democratic opponent, Dafna Michaelson Jenet, has raised more than three times the money Windholz has. Windholz’ biggest contributor has been the Adams County Republican Party, about $4,000; with another $400 from the Colorado Republican Party.

Straayer sees Windholz as the other Republican incumbent most at risk of defeat.

House District 47, Pueblo County

Democrats hold a small advantage of about 1,000 voters in voter registration over Republicans. Yet Navarro-Ratzlaff is the only Republican in a competitive seat to beat her Democratic opponent in fundraising; she has so far taken in $103,430 to Muñoz’ $34,262.

“I’m impressed with her ability to win in a Democratic district,” Straayer said, but if Trump does damage down-ballot, the Democrat could take it. Navarro-Ratzlaff has been one of the most high-profile Republicans in Colorado to back Trump.

It’s not all about the money. How will turnout affect the House?

Former GOP Chair Ryan Call told The Colorado Independentthat the state and county parties haven’t been helping with get-out-the-vote efforts or push GOP voter registration.

“There has not been the heavy lifting from the state GOP or county parties this election cycle that was seen in the past,” Call said. As a result, it’s falling on the candidates themselves or the outside groups to push turnout, he explained.

Call added that the GOP has not responded to the shift in voter registration to Democrats.”That failure in many ways will have a profoundly negative impact” on key races, he said.

But Call believes that Republican candidates will benefit from last-minute voters. He believes a lot of Republicans have been sitting on their ballots, “waiting to see how the presidential race will shake out. That will help down-ticket races,” he said.

Strayyer says a surge in ballot returns by Republicans near Election Day will impact races all the way down the ballot, he said. He also sees possibilities for Republican pickups in the Nelson/Gray race in Broomfield and the Brown/Bridges race in Arapahoe County.

Dumm thinks who wins the state House simply will come down to who worked the hardest. “In this cycle, Democrats would be favored, as long as they’re knocking on doors and haven’t done things to anger voters.” She is among those who think the Dems will keep the House, in part, because of the Trump effect.

“If I were in the legislature, I wouldn’t want to be running on a Republican ticket,” she said.